The present invention relates generally to a method for preparing aluminum nitride (AlN), and more particularly to a combustion synthesis method for preparing the powdery aluminum nitride.
Having a high thermal conductivity, a high electrical resistivity, a good mechanical strength, and a good oxidation and thermal-shock resistance, AlN becomes a very important ceramic material in industrial applications. It can be used for high-performance electronic substrate material, optical lenses, cutting tools, heat sinks, and many high-temperature structure materials.
The manufacturing methods for AlN include:
1) the gas phase reaction method, e.g., 
2) the direct nitridation method, e.g., 
3) the carbothermal reduction-nitridation method, e.g., 
4) the combustion synthesis method.
The gas reaction method is not suitable for mass production of AlN in industry because of the high cost and low productivity involved.
The direct nitridation of Al and the carbothermal reduction-nitridation of powdery Al2O3 methods in commercial form require a process executed under a high temperature and a long period of time, e.g., 5 hours, to fully complete the reaction, which can thus result in common disadvantages including a greater energy consumption and a slow manufacturing rate.
In comparison with other methods, the combustion synthesis method is a new method used to synthesize ceramic materials by self-propagation combustion reactions. The advantages achieved thereby include that it has a fast reaction rate, a less energy consumption and a simple manufacturing process and that it can be used for mass production.
Prior arts for the synthesis of aluminum nitride by the combustion synthesis method are described hereinbelow:
(1) Japan Patent Application Laid-Open No. 63-274605
Mix aluminum, aluminum nitride, and one powder selected from the group consisting of calcium carbonate (CaCO3), calcium nitrate (Ca(NO3)2), yttrium oxide (Y2O3), barium carbonate (BaCO3), barium nitrate (Ba(NO3)2), yttrium nitrate (Y(NO3)3), cerium oxide (CeO2), and yttrium oxalate hydrate (Y2(C2O4)2. 8H2O), according to an appropriate proportion, and then press the mixture into a suitable shape. Burn the cake by using an electric plate in a nitrogen atmosphere at 50 atm to form an aluminum nitride powder.
(2) Japan Patent Application Laid-Open No. 64-76906
Mix aluminum with aluminum nitride according to an appropriate ratio to be loaded in a porous refractory container. The assembly is heated by using an electric wire to form an aluminum nitride powder.
(3) Japan Patent Application Laid-Open No. 64-76905
Powders of aluminum and sodium nitride (NaN3) or other nitrogen-containing solid compound (such as potassium nitride (KN3), barium nitride (Ba3N2), etc.) are mixed according to an appropriate ratio and then placed in a refractory container. An igniting agent is placed on the powder mixture, and the refractory container is then placed into an electric oven which will be soon filled with nitrogen gas at a pressure of 10 kg/cm2. Prior to the igniting the reaction, the reactants are heated in the electric oven and then ignited with an electric wire to carry out the combustion reaction and form an aluminum nitride powder.
(4) U.S. Pat. No. 5,460,794
An aluminum powder and a solid nitrogen-containing compound are used as the raw material and molded after mixing. The blank is fully wrapped with an igniting agent and placed in an enclosed container filled with nitrogen which is ignited by heating the igniting agent with heating elements to form an aluminum nitride powder.
(5) U.S. Pat. No. 5,453,407
An aluminum powder and a solid-state nitrogen source are used as the raw material which is added with an ammonium halide powder. The mixture of the three is molded and fully wrapped with an igniting agent. The blank is placed in an enclosed container filled with nitrogen which is ignited by heating the compact by passing an electric current through the heating coil to form an aluminum nitride powder.
(6) U.S. Pat. No. 5,846,508
An aluminum powder and an ammonium halide salt are used as the raw material which are molded into a tablet or placed in an open or porous refractory container and then placed in an enclosed container filled with nitrogen. The combustion reaction is ignited to form an aluminum nitride powder.
(7) U.S. Pat. No. 5,649,278
An aluminum powder is used as a reactant and diluted with 20-80 wt % (based on the total weight of the aluminum powder and the aluminum nitride powder) of an aluminum nitride powder. The resultant mixture is poured into a graphite crucible or a refractory container made of oxide, ceramic, etc. The powder mixture has a bulk density of from 0.5-1.5 g/cm3. The container is placed in a reactor filled with nitrogen at 0.75-30 atm and the combustion reaction is ignited by directly heating the powder to form an aluminum nitride powder.
The key factors of synthesizing the aluminum nitride powder by combustion reaction are: (1) how to provide a sufficient amount of nitrogen; (2) how to prevent agglomeration of molten aluminum powder; and (3) how to achieve a complete reaction.
If a nitrogen gas is used as the nitrogen source, according to M. Costantino and C. Firpo. (J. Mater. Res. 6 : 2397 (1991)), the pressure needs to be upto 1000 atmosphere before the reaction can take place. The process used in the above-mentioned JP Laid-Open No. 63-274605 uses a pressure of 50 atm which is rather high and will cause an increase in the costs in facilities and operation thereby increasing the complexity and danger in the operation thereof.
If liquid nitrogen is used as the nitrogen source (as mentioned in the above JP 64-76906) without the need of using high pressure, the cost and the complexity of operation of the facilities will be increased because the boiling point of the liquid nitrogen is too low.
If a solid-sate nitrogen source is used as the nitrogen source (such as the above-mentioned JP 64-76905, U.S. Pat. No. 5,460,794 and U.S. Pat. No. 5,453,407), even without the use of high pressure, said solid-state nitrogen source needs to be a compound that is easily thermally decomposable for the combustion reaction to advance. Under such a circumstance, the reaction setup needs to be designed properly (such as wrapped with an igniting agent) so that the nitrogen gas generated by the thermal decomposition of the solid nitrogen source can promptly react with the aluminum powder. Otherwise, the escape of nitrogen gas will occur and hinder the performance of the reaction.
If an ammonium halide salt is added into the aluminum powder (e.g. the above-mentioned U.S. Pat. No. 5,846,508), the reaction process will generate by-products such as HCl, NH3, NH4Cl, H2 or Cl2, etc., and increase the complexity and operation costs of the downstream processes even though a high yield can be achieved at a low nitrogen pressure.
If the aluminum powder is added with a diluent such as aluminum nitride, etc. and the density of the mixture powder is 0.5-1.5 g/cm3 (such as mentioned in U.S. Pat. No. 5,649,278), the process requires a homogeneous mixing of the aluminum powder and the aluminum nitride and the content of the diluent needs to be upto 30 wt % even though said process can prevent the agglomeration of the aluminum powder, maintain a proper circulation of the nitrogen gas, and reach a high conversion rate. Such a process also has a high complexity, a high operation cost, and a reduced production efficiency (based on the amount of the aluminum nitride that can be synthesized by the feed per unit weight). Furthermore, the density of the powdered feed needs to be 0.5-1.5 g/cm3, which is not suitable for a packing density higher than ( greater than 1.5 g/cm3) or lower than ( less than 0.5 g/cm3) of the aluminum powder (and the aluminum nitride powder), and limits the selection scope of the raw material.
The present invention provides an efficient new technology to synthesize an aluminum nitride of excellent properties by making improvements to the conventional combustion synthesis methods.
To achieve the above objective, a process for preparing an aluminum nitride according to the present invention comprises the following steps:
(a) loading a refractory container having an opening with an aluminum-containing raw material powder having a packing density of from 0.1 to 1.6 g/cm3; carrying out either one or both of the following Step (a-1) and Step (a-2) if the packing density of the raw material powder is higher than 0.8 g/cm3; optionally carrying out Step (a-1) or Step (a-2) if the packing density of the raw material powder is not higher than 0.8 g/cm3:
(a-1) vertically disposing one or plural aluminum pipes with perforations on the pipe wall thereof in said raw material powder in said refractory container;
(a-2) placing a layer of an initiator on top of the raw material powder in said refractory container, said initiator comprising an aluminum powder and an additive capable of preventing agglomeration of molten aluminum powder;
(b) placing said loaded refractory container in a nitrogen atmosphere;
(c) introducing nitrogen gas into said loaded refractory container through a bottom or a side near the bottom of said loaded refractory container, so that the introduced nitrogen gas can flow through the raw material powder and out of the loaded refractory container through the opening thereof; and
(d) heating the raw material powder or the initiator in the loaded refractory container until a self-combustion of the raw material powder or the initiator is brought about.
The invented process optionally comprises the following steps:
(e) after cooling, grinding the combustion product generated in Step (d) into a powder.
The raw material powder of Step (a) can be a pure aluminum powder, an aluminum-containing alloy powder, a mixture of a pure aluminum powder and other elements, scraps of aluminum which are generated in fabrication of aluminum products, or scraps of aluminum alloy which are generated in fabrication of aluminum alloy products. Preferably, the aluminum-content of the raw material powder which is not pure aluminum powder is higher than 50% by weight. When the purity of aluminum is higher, the purity of the AlN product will be higher. The products will be composites composed of AlN, compounds formed of nitrogen and impurities, and residual impurities, when the purity of aluminum is lower,.
The refractory container of Step (a) can be a graphite crucible, or a crucible made of ceramics of AlN, Si3N4, Al2O3, ZrO2, WC, etc. The packing density of the raw material powder can be calculated by the weight and volume of the raw material powder measured in the blank experiment conducted in advance. The aluminum pipe having perforations in Step (a-1) can be an aluminum pipe which is made integrally to have holes on its wall, a perforated aluminum pipe, an aluminum pipe having perforations formed by wrapping one layer of punched aluminum foil, or an aluminum pipe having perforations formed by wrapping one or several layers of aluminum foils and punching with holes thereon. The height of the aluminum pipe is determined by the height of the raw material powder loaded. That is, one end of the aluminum pipe can be placed on the bottom of the crucible; while the other end just juts out from the top of the raw material powder loaded. The inside diameter of the aluminum pipe can be ranged from 1 mm to half of the inside diameter of the crucible, preferably 2-5 mm. The wall thickness of the aluminum pipe can be 0.01 to 0.5 mm, preferably 0.05 to 0.2 mm. The aluminum pipe should have a wall thickness, in principle, so that the pipe will not be crashed and will remain as a gas tunnel after the raw material powder is loaded, and thus it can be completely burned into aluminum nitride powder. The perforations may have a diameter ranging from 0.001 to 0.3 mm, preferably 0.02 to 0.2 mm. The density of the perforations on the aluminum pipe wall can be 1-50%, preferably 5-30%, of the total surface area of the aluminum pipe having no perforation. The number of aluminum pipes should be such that the total cross-sectional area of the aluminum pipes is 1 to 50%, preferably 5 to 20%, of the cross-sectional area of the crucible.
The initiator of Step (a-2) can be a mixture powder of an aluminum powder and an additive. Said additive can be selected from any group of the following four groups (one compound or a mixture of more compounds in the same group) or a mixture of compounds from more than one groups (more than one compounds can be selected from the same group): (i) ammonium halide, i.e. NH4F, NH4Cl, NH4Br and NH4I; (ii) NHxxe2x80x94 or halogen-containing compounds degradable or gasifiable below the melting point (660xc2x0 C.) of aluminum, e.g. urea [CO(NH2)2], NH2CO2NH4, ammonium carbonate [(NH4)2CO3], NH4HCO3, HCOONH4, N2H4. HCl, N2H4. HBr, N2H4. 2HCl, aluminum chloride (AlCl3), aluminum bromide (AlBr3) and ferric chloride (FeCl3) etc.; (iii) aluminum nitride powder; (iv) other powders with a high melting point, e.g. carbon powder, BN, TiN, SiC, Si3N4 etc. The additive is 0.01 to 100% by weight of the initiator, preferably 0.05 to 60%. The layer of the initiator is 1xcx9c10 mm on top of the raw material powder, preferably 2xcx9c5 mm.
Preferably, the nitrogen atmosphere in Step (b) is formed by vacuuming an airtight chamber and introducing nitrogen gas into the vacuumed chamber, wherein a loaded refractory container is placed in said airtight chamber in advance. Said airtight chamber can be a reactor capable of withstanding a high pressure. The pressure of nitrogen gas in said airtight chamber after introducing nitrogen gas can be 0.1 to 30 atm, preferably 0.5 to 5 atm.
The nitrogen gas in Step (c) is introduced through a nitrogen-conveying pipe connected to a hole formed on the bottom, or the side and near the bottom of the refractory container. Preferably, the refractory container is provided with a gas chamber at the bottom thereof, and the gas chamber has a porous top on which the raw material powder is loaded. When carrying out Step (a-1), the lower end of said perforated aluminum pipes stand on said porous plate. The nitrogen gas introduced first flows into said gas chamber, through said porous plate, upwards through said raw material powder, and out from the top surface thereof, such that the nitrogen gas can uniformly flow through the cross-section of said raw material powder. The porous plate of said gas chamber can be made of graphite or ceramics of AlN, Si3N4, Al2O3, ZrO2, WC, etc.
Under certain operation conditions, the product obtained from the reaction will adhere to the wall of the refractory container and is difficult to be removed. Therefore, a layer of aluminum nitride powder can be placed between the raw material powder and the container inner wall while loading said raw material powder.
The reactants can be ignited by using an electric heater made of tungsten filament, tungsten tape, graphite tape or plate, wherein the electric heater is moved closer to the top surface of said raw material powder or the initiator layer. Alternatively, the top surface of said raw material or said initiator is irradiated by a laser, microwave or infrared beam. In fact, any means which can heat the top surface of said raw material powder or the initiator layer to 700 to 1600xc2x0 C. are all applicable in the present invention.
The shape of the aluminum nitride powder (morphology, i.e. size and shape of the particles, etc.) produced according to the method of the present invention varies with the combustion temperature. Therefore, the combustion temperature can be used to control the shape of the particles. The control of the combustion temperature can be carried out by simply adding an appropriate amount of the diluent in the raw material powder in Step (a). The amount of addition can be 0.01xcx9c60% of the total weight of the raw material powder and the diluent, where the diluent can be a ceramic powder of AlN, Si3N4, TiN, BN, SiC, Al2O3, ZrO2, TiO2, SiO2 etc. If AlN is used as the diluent, the product is a pure AlN; while if added with other ceramic powder as the diluent, the product is a composite material of AlN and the diluent.
The particle shape and porosity of the aluminum nitride powder according to the present invention can be controlled by the addition of an expanding agent. The expanding agent itself or its gasification or decomposition under heating can increase the porosity of the raw material powder. Said expanding agent can be selected from any group of the following three groups (one material or a mixture of more materials in the same group) or a mixture of materials from more than one groups (more than one materials can be selected from the same group): (I) ammonium halide, i.e. NH4F, NH4Cl, NH4Br and NH4I; (II) NHxxe2x80x94 or halogen-containing compounds degradable or gasifiable below the melting point (660xc2x0 C.) of aluminum, e.g. urea [CO(NH2)2], NH2CO2NH4, ammonium carbonate [(NH4)2CO3], NH4HCO3, HCOONH4, N2H4. HCl, N2H4. HBr, N2H42Cl, aluminum chloride (AlCl3), aluminum bromide (AlBr3) and ferric chloride (FeCl3) etc.; and (III) aluminum foil balls of about 0.1 to 2 mm in size formed by wrapping a small piece of aluminum foil. The amount of the expanding agent is 0.01xcx9c10% of the total weight of said aluminum powder and said expanding agent.
The characteristic peaks of aluminum can not be observed in the XRD (X-ray diffraction) analysis of the product synthesized according to the present invention. Trace amount of the residual aluminum can be removed from the product synthesized according to the present invention by acid cleaning. The type and quantity of the impurities contained in the product depend on the aluminum content of the raw material powder and the purity thereof. Since the high combustion temperature tends to remove the volatile impurities, the amount of impurities contained in the product is usually lower than the amount of the original impurities contained in the raw material powder. In an example of the present invention, the combustion product was poured into an acid solution (an aqueous solution containing 8% by weight of HCl and 1% by weight of HF) to dissolve the residual aluminum after grinding; filtered, cleaned with a deionized water and dried to obtain the aluminum nitride product. There should have a small amount of aluminum nitride lost during the acid washing process. However, the extent of such a loss is difficult to assess. Hence, the difference between the weight of the product before the acid washing and the weight of the aluminum nitride product obtained is considered as the content of un-reacted aluminum in the combustion product. In the present invention, the term xe2x80x9cyieldxe2x80x9d is defined as the weight of aluminum in the synthesized aluminum nitride product divided by the weight of aluminum in the raw material powder including, if any, an aluminum nitride powder added as a diluent. Under most operating conditions, the yield of the method of the present invention is higher than 95%. The products mostly are mainly in the form of particles, while rod-like, branch-like, or fibrous forms may also occur. The products synthesized under most operating conditions of the method of the present invention, are nearly all in the form of powder less than 20 xcexcm in size after being ground with a planetary mill (at 400 rpm, for 20 min, with grinding balls made of aluminum oxide). The products are in white, maize or yellow, and are all AlN from an XRD analysis.
In the method of the present invention, if Step (a-1) or Step (a-2) is not carried out when the packing density of the raw material powder is higher than 0.8 g/cm3, the combustion reaction may still be able to undergo. However, the yield is always not high, i.e lower than 95%, and inversely proportional to the packing density of the raw material powder. The combustion reaction can not be ignited, if the packing density of the raw material powder is too high. On the other hand, if Step (a-1) or Step (a-2) are adopted when the packing density of the raw material powder is lower than 0.8 g/cm3, the yield is equally high as the one without adopting Step (a-1) or Step (a-2) (higher than 95%, and higher than 98% under most circumstances).
The major differences between the process according to the present invention and the conventional processes in synthesizing a powder of aluminum nitride by a combustion synthesis method are:
(1) The Introduction of Nitrogen Gas During the Reaction and Allowing the Nitrogen Gas to Flow Through the Raw Material Powder:
When the technique according to the present invention is used to produce aluminum nitride, during the reaction stage, a nitrogen gas is constantly introduced into the reaction system, and flows through the raw material powder uniformly. The functions provided by this are: (1) fully supplying the nitrogen required for forming nitride; (2) maintaining the raw material powder at a loosened state and preventing the agglomeration of the molten raw material powder; (3) when the combustion wave is propagating downwards from the top surface of the raw material powder, the raw material powder located below the combustion wave will first form aluminum nitride on the outer surface thereof due to a sufficient supply of nitrogen gas and the heating of the combustion wave, where the formation of such an aluminum nitride layer can prevent the agglomeration of the molten raw material powder, thereby enabling the nitrogen gas to be fully supplied to different parts of the reactants so that a complete reaction can be achieved.
(2) The Use of a Porous Aluminum Pipe and/or an Initiator for a Raw Material Powder with a High Packing Density:
If the packing density of the raw material powder is higher than 0.8 g/cm3, the combustion reaction is not easily ignited, due to a poor gas flowing homogeneously through the raw material powder, thereby causing a lower product yield. Under such a circumstance, the problems can be resolved by (1) vertically disposing perforated aluminum pipes in the raw material powder; and/or (2) disposing a layer of an initiator on top of the raw material powder. The perforated aluminum pipes can assist the flow of the nitrogen gas. The additives in the initiator can prevent the agglomeration of the molten raw material powder. Therefore, a constant flow of the nitrogen gas can be maintained and a high conversion rate can be achieved.
(3) Without the Need of Adding a Diluent or an Additive Into the Raw Material Powder:
If the packing density of the raw material powder is lower than 0.8 g/cm3 and pure aluminum powder is used as the raw material powder, an extremely high conversion rate ( greater than 95%) can be reached without the need of adding an ammonium halide salt as disclosed in U.S. Pat. No. 5,846,508, as well as without the need of adding an aluminum nitride powder as stated in U.S. Pat. No. 5,649,278. Furthermore, if the packing density of the raw material powder is higher than 0.8 g/cm3, an extremely high conversion rate ( greater than 95%) can still be reached without the need of adding any additive or diluent by adopting the perforated aluminum pipes or the initiator layer as mentioned hereinbefore. The present invention, however, does not preclude the possibility of adding any additive or diluent mentioned for the sake of controlling the combustion temperature.
(4) Wide Range in the Applicable Types of the Raw Material Powder and the Packing Density:
A wide variety of the types of the raw material powder, including dense particles, porous particles, fluffy particles, globular particles, irregular particles, pellet-like, flake type, etc., are applicable as the reactant of the present invention. The packing density of the aluminum powder can range from 0.1 to 1.6 g/cm3 in the present invention without the restriction that the density of the reactants needs to be within 0.5 to 1.5 g/cm3 as disclosed in U.S. Pat. No. 5,649,278.